Electrical measurements of membrane potential and resistance of neuroblastoma and cultured muscle cells have been made using intracellular microelectrodes while the cells are undergoing immunologic attack by antibody and complement and by killer T lymphocytes. Membrane damage by complement is indicated by a rapid loss of membrane potential and resistance which occurs 20-30 min prior to cell death measured by dye uptake or phase microscopy. Alloantigen sensitized T lymphocytes specifically cause membrane depolarizations of 20-40 mv lasting 3-30 sec within minutes after contact with the target cells with only minor changes in resistance. In some cases a complete and irreversible depolarization followed after 1-3 hrs. Another approach to examining such membrane damage has been to study the escape of markers from resealed erythrocyte ghosts being attacked by various lytic agents. A series of fluorescent proteins and dextrans of various molecular weights was resealed inside the ghosts; a sieving effect was revealed when small macromolecules but not large ones were released by complement and by an antibody dependent lymphocyte mediated process. Aqueous channels through the membrane having diameters of 50 A and 140 A for complement and lymphocyte killing respectively are suggested from the size cut-off of the protein markers.